High-density energy storage by intercalation of MXene phase layers

Published on October 4th, 2013 | By: Eileen De Guire

Illustration of intercalation of cations from aqeuous solution into 2D MXene material, such as titanium carbide. (Credit: Drexel University)

As researchers gain traction with renewable and alternative energy generation technologies, a parallel effort to improve energy storage technologies also is underway. Batteries are chief among these, although there is also interest in other systems like batteries’ cousins, supercapacitors, and flywheels.

In the battery family, lithium-ion batteries remain top dog, and a recent paper in Science shows there may be a few new tricks for the old dog to learn.

The paper is published by Drexel University’s nanotechnology duo Yury Gogotsi and Michel Barsoum, who discovered the two-dimensional MAX phases and have made a systematic and productive study of them for the better part of the last two decades. (For an overview, see the April 2013 issue of the ACerS Bulletin.) MAX phases are two-dimensional transition metal-metal-carbides or nitrides. Example compounds are TiAlC and TiNbC.

Last May we reported that the team, inspired by work on graphene, had discovered a way to use exfoliation to remove the “A” group metal, leaving behind a two-dimensional MX phase (called “MXene” to tie composition to structure, i.e, graphene structure). Computer simulations indicated that large energy storage capacities might be possible, and followup experiments confirmed that very large amounts of lithium ions could be stuffed into the exfoliated material at very high charging rates.

With such promising preliminary results in hand, the team has begun to explore in depth the mechanisms of ion storage, in particular the intercalation process by which MXenes incorporate ions between their layers. They were interested in discovering whether spontaneous intercalation of ions from aqueous solutions was possible, as it had not been predicted from theoretical calculations, nor had it been demonstrated.

The new paper reports on “spontaneous intercalation of cations from aqueous salt solutions” of Li+ as well as Na+, K+, NH4+, Mg2+, and Al3+. In fact, their results “show that a variety of cations of various charges and sizes can readily intercalate” from aqueous solutions. Intercalated MXenes exhibited exceptionally large storage capacities up to 350 F/cm3. In a press release, Barsoum says, “This capacity is significantly higher than what is currently possible with porous carbon electrodes. In other words, we can now store more energy in smaller volumes, an important consideration as mobile devices get smaller and require more energy.”

The paper reports on TiC MXene, but the team has identified nine MXene phases so far. In the press release, Gogotsi says there are likely many more similar phases yet to be discovered. “So even the impressive capacitances that we are seeing here are probably not the highest possible values to be achieved using MXenes,” he says. “Intercalation of magnesium and aluminum ions that we observed may also pave the way to development of new kinds of metal ion batteries.”

Besides discovering new materials, the team has also made some some interesting developments regarding new forms of materials. In particular, the idea of flexible, paperlike forms of materials is broadening the way researchers think about functional materials, and whom they think about them with. About a year ago, for example, we reported on new work in Germany on flexible nanofiber vanadium oxide paper. Similarly, Gogotsi and Barsoum say they can make MXene “papers” which are flexible and could be used in flexible devices or possibly wearable devices. To this end, they have started working with other Drexel faculty to explore the intersection between materials science and material objects, that is, science and art.